According to the World Bank, U.S. per capita electrical consumption has exceeded 13,000 kW. Throughout the world electricity consumption was over 20,000,000 GWh/yr in 2009. Traditional electrical generation, through use of coal and other fossil fuels, has increased, due in part to the dwindling supply of resources. Low-efficiency peaking power plants, such as gas turbines, are still employed for meeting high peak energy demands. However, the electrical generation industry has begun providing lower cost, off-peak rates to change consumer usage to times where electricity may be produced at much lower marginal cost.
Ecologically friendly electrical generation has become more common, as a means of reducing carbon emissions and reducing reliance on nonrenewable resources. Photovoltaic cells and concentrating Solar Power (CSP) are competing solar power generation technologies. CSP systems use lenses or mirrors to focus a large area of sunlight into a small beam, which is used as a heat source for a conventional power plant. A working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.
Thermal energy storage technology saves energy, provides economic benefits, and makes possible the use of periodic energy sources, such as solar energy. Solar energy is an inexhaustible source of future energy needs with a minimum of adverse environmental consequences (Goswami, et al., 2000, Principles of Solar Engineering, Taylor and Francis Group, New York). The energy storage is one of the best possible solutions to reduce the mismatch between supply and demand. Most current solar heating systems have storage for a few hours to a day's worth of energy collected, by using thermal energy storage materials (TESMs). Thermal energy storage (TES) helps overcome the intermittency of the solar resource for concentrating solar power (CSP) plants (Dincer and Dost, 1996, “A perspective on thermal energy storage systems for solar energy applications,” International Journal of Energy Research, 20(6), pp. 547-557). TES systems contains a thermal storage mass, and can be based on latent, sensible and thermo-chemical energy storage (Sharma, et al., 2009, “Review on thermal energy storage with phase change materials and applications,” Renewable and Sustainable Energy Reviews, 13(2), pp. 318-345; Gil, et al., 2010, “State of the art on high temperature thermal energy storage for power generation. Part 1—Concepts, materials and modellization,” Renewable and Sustainable Energy Reviews, 14(1), pp. 31-55).
Among these energy storage types, latent heat storage in phase change materials (PCMs) is very attractive because of high storage capacity and charging/discharging heat at a nearly constant temperature, require little maintenance, can be produced at a range of temperatures, and are easy to integrate into a power plant (Abhat, “Performance studies of a finned heat pipe latent thermal energy storage system,” Proc. Mankind's future source of energy; Proceedings of the International Solar Energy Congress, Pergamon Press, Inc., New Delhi, India, pp. 541-546). There are several studies on storing solar thermal energy using PCMs (Zalba, et al., 2003, “Review on thermal energy storage with phase change: materials, heat transfer analysis and applications,” Applied Thermal Engineering, 23(3), pp. 251-283; Al-Jandal and Sayigh, 1994, “Thermal performance characteristics of STC system with Phase Change Storage,” Renewable Energy, 5(1-4), pp. 390-399; Baran and Sari, 2003, “Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system,” Energy Conversion and Management, 44(20), pp. 3227-3246; Fouda, et al., 1984, “Solar storage systems using salt hydrate latent heat and direct contact heat exchange—II Characteristics of pilot system operating with sodium sulphate solution,” Solar Energy, 32(1), pp. 57-65; Morrison and Abdel-Khalik, 1978, “Effects of phase-change energy storage on the performance of air-based and liquid-based solar heating systems,” Solar Energy, 20(1), pp. 57-67; Rabin, et al., 1995, “Integrated solar collector storage system based on a salt-hydrate phase-change material,” Solar Energy, 55(6), pp. 435-444; Velraj, et al., 1999, “Heat Transfer Enhancement in a Latent Heat Storage System,” Solar Energy, 65(3), pp. 171-180; Medrano, et al., 2010, “State of the art on high-temperature thermal energy storage for power generation. Part 2—Case studies,” Renewable and Sustainable Energy Reviews, 14(1), pp. 56-72; Jotshi, et al., 1992, “Solar thermal energy storage in phase change materials,” SOLAR '92: American Solar Energy Society (ASES) Annual Conference Cocoa Beach, Fla., pp. 174-179). Stored solar thermal energy has the potential to provide cheaper peak-demand power than any other energy source. In addition, there are also a number of seasonal thermal stores, which store summer energy for space heating during winter.
PCMs store heat using solid-solid, solid-liquid, solid-gas and liquid-gas phase change, though only solid-liquid change is used in PCMs for electrical generation and thermal energy storage. Solid-liquid PCMs increase in temperature as they absorb heat, until PCM reaches the phase change temperature (melting temperature). At the phase change, the PCM absorbs large amounts of heat with minimal temperature change, until the material has transformed phase. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its considerable amount of latent energy. PCMs are widely used in the art because of the high energy storage density associated with the change of phase. PCM storage systems also have an advantage of being capable to operate with small temperature differences between charging and discharging.
In a latent heat energy storage system, selection of the PCM is a very important task. Most investigations are focused on salt hydrates, paraffin's, non-paraffin's, inorganic acids, clathrates and eutectic organic and inorganic compounds (Lane, 1986, Solar Heat Storage: Latent Heat Materials, CRC Press, Inc, Boca Raton, Fla.). For example, Lane (Lane, 1986, Solar Heat Storage: Latent Heat Materials, CRC Press, Inc, Boca Raton, Fla.) has compiled a list of the common PCMs for various applications. Inorganic compounds, in general, have a higher latent heat per unit volume and are non-flammable, and have lower costs in comparison to organic compounds (Tyagi, et al., 2011, “Development of phase change materials based microencapsulated technology for buildings: A review,” Renewable and Sustainable Energy Reviews, 15(2), pp. 1373-1391; Agyenim, et al., 2010, “A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS),” Renewable and Sustainable Energy Reviews, 14(2), pp. 615-628).
Many PCM systems rely on molten salt to retain the temperature. Molten salt can be employed as a thermal energy storage method to retain thermal energy collected by a solar tower or solar trough so that it can be used to generate electricity in bad weather or at night. With concentrated solar power, the PCMs have been successfully utilized in electricity generation, to allow solar power to provide electricity on a continuous basis, such as Solar One, Two, and Tres. These molten salts, like potassium nitrate, calcium nitrate, sodium nitrate, lithium nitrate, absorb and store the heat energy that is later released when electricity is needed by pumping the hot salt to a conventional steam-generator to produce superheated steam for a turbine/generator.
However, there are some problems associated with PCMs. Salt based PCM solutions must be encapsulated to prevent water evaporation or uptake. One of the most significant drawbacks is that these materials have very low heat transfer characteristics, and tend to solidify at the edges of the encapsulating container and preventing effective heat transfer. Moreover, inorganic PCMs, in general, have low thermal conductivity (0.1-0.6 W/m. K), leading to low heat transfer rates and oxidation on exposure to heat transport medium (air or heat transfer fluids like oils). In order to overcome such problems, heat transfer enhancement techniques, such as use of extended surfaces and dispersion of high conductivity materials, have been identified and applied (Jegadheeswaran and Pohekar, 2009, “Performance enhancement in latent heat thermal storage system: A review,” Renewable and Sustainable Energy Reviews, 13(9), pp. 2225-2244). Another technique to overcome the low heat transfer is to encapsulate the PCM within a secondary supporting structure, and use of these capsules in a packed/fluidized bed heat exchanger (Hawlader, and Zhu, 2000, “Preparation and Evaluation of a Novel Solar Storage Material: Microencapsulated Paraffin,” International Journal of Solar Energy, 20(4), pp. 227-238).
No commercially available heat storage products possess heat transfer enhancement capabilities which would improve performance of low temperature devices. Enhancement of heat transfer in heat storage devices with a PCM typically use fins and Lessing rings from various materials and carbon fibers (Kenisarin and Mahkamov, 2007, “Solar energy storage using phase change materials,” Renewable and Sustainable Energy Reviews, 11(9), pp. 1913-1965). The disadvantage of their heat exchanger development is increasing the cost and complexity of thermal energy storage devices. In order to solve these problems, both material investigation and heat exchanger development were performed. Based on the investigation of studies focused on both the cases, it was observed and reported that one of the effective techniques is to encapsulate the PCM within a secondary supporting structure, and apply the packed/fluidized bed heat exchanger for better heat transfer enhancement (Hawlader and Zhu, 2000, “Preparation and Evaluation of a Novel Solar Storage Material: Microencapsulated Paraffin,” International Journal of Solar Energy, 20(4), pp. 227-238). Since the progress of latent heat storage systems mainly depends on assuring a high effective heat transfer rate to allow rapid charging and discharging, the required heat transfer surfaces should be large to maintain a low temperature gradient during these processes. This can be achieved efficiently through the macroencapsulation.
Macroencapsulated PCMs refer to capsules larger than 1 mm (Li, et al., 2012, “Fabrication and morphological characterization of microencapsulated phase change materials (MicroPCMs) and macrocapsules containing MicroPCMs for thermal energy storage,” Energy, 38(1), pp. 249-254). PCM macrocapsules are generally made by filling the PCM in preformed shells such as tubes, pouches, spheres, panels or other receptacles and sealing them (Cabeza, et al., 2011, “Materials used as PCM in thermal energy storage in buildings: A review,” Renewable and Sustainable Energy Reviews, 15(3), pp. 1675-1695). The most cost-effective containers are plastic bottles (high density and low density polyethylene and polypropylene bottles for low temperature storage), tin-plated metal cans and mild steel cans (Regin, et al., 2008, “Heat transfer characteristics of thermal energy storage system using PCM capsules: A review,” Renewable and Sustainable Energy Reviews, 12(9), pp. 2438-2458; Bauer, et al., 2012, “Characterization of Sodium Nitrate as Phase Change Material,” International Journal of Thermophysics, 33(1), pp. 91-104; Farid, et al., 2004, “A review on phase change energy storage: materials and applications,” Energy Conversion and Management, 45(9-10), pp. 1597-1615; Chou, T. P., Chandrasekaran, C., Limmer, S., Nguyen, C., and Cao, G. Z., 2002, “Organic-inorganic sol-gel coating for corrosion protection of stainless steel,” Journal of Materials Science Letters, 21(3), pp. 251-255).
While these problem may be overcome by microencapsulation or molecular-encapsulation of PCMs, the encapsulation process may be expensive and difficult. Therefore, what is needed is a thermal energy storage material composition which permits effective heat transfer, without unyielding encapsulation.